There are a few things to consider if you don't have a current version of BS7671 with the relevant guidance notes and building regulations relevant to your work.
If Things go Wrong:
If you did some work resulting in a fire/electrocution and you where sent to court on trial, you'd need to prove you are competent and did everything reasonably possible. The courts will always refer to BS7671 and it's relevant guidance-notes. "Do you own a copy? " and if not "how can you be sure your work was compliant?" and if it's not compliant "why was it not in compliance, was it a calculated deviation from regulations or negligence?" - this wouldn't end well.
Deviations from Regulations:
Some highly-educated and experienced designers will occasionally deviate from the regulations during their design, albeit with good reasons. They would make sure to document these deviations with extensive calculations and considerations for safety. This is outside of the scope of most electricians (including myself) because if something where to happen, you'd need to explain why you deviated from the regulations and what makes you competent in such a decision (I don't have a degree, or decades of design experience) so it's not normally worth it.
Notifiable Work:
If you're a member of a competency scheme, they may insist you own a current version of BS7671 (plus relevant guidance notes, OSG, Building Regulations, etc - which they will test you're knowledge and check previous work you've done). They may also insist you follow the regulations in order for them to allow you to self-certify you're work.
Alternatively if you use a building inspector, they will be following the guidance of BS7671 themselves to check your work and may not agree with any alterations.
To summarise, you don't (technically) need BS7671, nor do you need to follow it's advice. But if you don't, make sure you are competent to make that decision and have a complete-knowledge of what it says, and document why with great detail.